Performance testing interview questions
// 38 QUESTIONS · UPDATED MAY 2026
Performance testing interview questions covering JMeter, k6, Gatling, load patterns, percentile analysis, and how to diagnose bottlenecks from real-world test data.
Showing 38 of 38 questions
- What is the difference between load testing and stress testing?Junior
Load testing measures behaviour at expected production traffic. Stress testing pushes well beyond that to find the breaking point and how…
- Why do you report p95 instead of average response time?Mid
Averages hide the long tail. A request that's fast for 95% of users but takes 8 seconds for the slowest 5% will look 'fine' on average bu…
- What are the key metrics in performance testing?Junior
Response time (usually as percentiles — p50/p95/p99), throughput (requests or transactions per second), error rate (% of failed requests)…
- How would you compare JMeter, k6, and Gatling at a high level?Junior
JMeter is GUI-driven Java, mature, plugin-rich, heavy on RAM. k6 is JavaScript, scriptable, CLI-first, modern, integrates well with CI. G…
- What's the difference between concurrency and arrival rate?Junior
Concurrency (closed model) holds N users active at once — if the system slows, fewer requests per second. Arrival rate (open model) injec…
- Walk through designing a load test for a checkout flow.Mid
Map the user journey end-to-end, parametrise with realistic data (carts, addresses, cards), mix transaction types by production ratio, mo…
- How do you handle dynamic data like CSRF tokens and session IDs in JMeter?Mid
Extract values from prior responses with regex/JSON/CSS extractors, store as JMeter variables, then reference them in subsequent requests…
- What are think times and why do they matter in load tests?Mid
Think time is the pause a real user makes between actions — reading a page, deciding, typing. Removing think times multiplies effective l…
- How do you parametrise test data in k6?Mid
Load CSV or JSON via SharedArray for memory efficiency across VUs, pick rows by VU/iteration index for uniqueness, pull secrets from envi…
- How do you set realistic SLOs for a load test?Mid
Start from production data — historical p95/p99 latencies and user-impact studies. Set SLOs tighter than current performance to drive imp…
- What's the difference between load testing and stress testing in how you actually execute them?Mid
Load runs at expected target RPS sustained for a long hold — pass/fail against SLOs, no surprises wanted. Stress ramps past target until…
- How would you use k6 thresholds to fail a CI pipeline on regression?Mid
Define thresholds per metric (p95 latency, error rate, custom metrics) — k6 exits non-zero on breach, failing the pipeline step. Persist…
- How do you correlate response time with backend resource saturation?Mid
Run the load test while collecting host and service metrics (CPU, memory, IO, DB pool, GC, queue depth) on synchronised timestamps. Plot…
- What's the difference between server-side and client-side performance metrics?Mid
Server-side measures backend behaviour (request latency, DB time, throughput) — load tools and APM see this. Client-side measures user ex…
- How do you isolate whether a slow response is the database, application, or network?Senior
Use distributed tracing to break the request into spans — DB query time, app compute time, network legs. APM (Datadog, New Relic) shows p…
- Walk me through how you'd plan capacity testing for a Black Friday spike.Senior
Model expected peak from historical data (e.g. 5x last year's peak), test 2x peak to verify headroom, run scenarios for spike, sustained,…
- How do you load test a system with heavy WebSocket or SSE traffic?Senior
Use k6's ws module or Gatling for WebSocket — open many concurrent connections, send/receive messages, measure connection-establishment t…
- How do you handle warm-up periods and cold-cache effects in load tests?Senior
Run an explicit warm-up stage at 10-20% of target load to populate caches, JIT-compile, and prime connection pools — discard those metric…
- How do you build a sustainable performance test suite that runs in CI without becoming a bottleneck?Senior
Tier the suite — fast smoke perf on every PR (under 5 min, narrow scope), full load nightly, soak weekly. Use thresholds that fail loud o…
- What's your approach to soak testing memory leaks over 8+ hour runs?Senior
Run sustained moderate load (30-50% of peak) for 8-24 hours while capturing heap, RSS, file descriptors, DB connection counts, and GC fre…
- How would you generate realistic test data at scale for a marketplace search load test?Senior
Sanitised production query logs are gold — anonymise PII, then replay actual query distributions. For synthetic generation, model Zipfian…
- How do you reproduce and root-cause a flaky performance result?Senior
Check noisy-neighbour effects (shared infra, CI runner contention), GC pauses, connection-pool warm-up, downstream rate limits. Re-run wi…
- How would you justify investment in a dedicated performance testing function?Lead
Tie to incidents avoided (cost of one Black Friday outage), customer impact (latency complaints, churn correlation with p95), and enginee…
- What is spike testing and how does it differ from stress testing?Junior
Spike testing applies a sudden, extreme burst of load for a short duration to see whether the system recovers gracefully. Stress testing…
- What is volume testing and when would you use it?Junior
Volume testing verifies that the system behaves correctly when it handles large amounts of data — not concurrent users, but data size. Us…
- What are Core Web Vitals and why do they matter for web performance testing?Junior
Core Web Vitals are Google's set of user-experience performance metrics: LCP (Largest Contentful Paint — load speed), INP (Interaction to…
- How do you design a realistic workload model for a load test?Mid
A workload model defines who your virtual users are, what they do, in what proportion, and at what pace. Derive it from production access…
- What is a ramp-up period in a load test and why does it matter?Mid
A ramp-up period gradually increases virtual users from zero to the target load over a defined time, rather than hitting full load immedi…
- What is a performance test baseline and how do you establish one?Mid
A baseline is a measured performance profile of the system at a known, stable state — usually the current production version. It sets the…
- How do you detect performance regressions between releases automatically?Mid
Compare each run's key metrics against the stored baseline using percentage-deviation thresholds. Fail the build when p95 or p99 exceeds…
- How do you test the performance impact of caching (Redis, CDN, database query cache) in a load test?Mid
Run the test in two modes: cold-cache (cache cleared before run) and warm-cache (cache seeded or pre-warmed). The difference in p95 betwe…
- How do you integrate Lighthouse into a CI pipeline to catch frontend performance regressions?Mid
Use the Lighthouse CI npm package to run Lighthouse against a running preview build on every PR. Define assertions for Core Web Vitals an…
- How does test environment fidelity affect your confidence in performance test results?Mid
A performance test run against an under-specced environment produces results that are valid only for that environment. Always document th…
- How do you generate distributed load when a single machine is not enough?Senior
Use your tool's native distributed mode: k6 Operator on Kubernetes, JMeter in distributed mode with controller + injectors, or Gatling En…
- How do you test API rate limiting under load?Senior
Test three scenarios: normal traffic below the limit (no 429s), burst traffic that hits the limit (429s appear at the expected threshold)…
- How do you test database query performance under realistic load?Senior
Enable slow query logging during load tests, use EXPLAIN ANALYZE on queries that appear in the slow log, and measure query latency at bot…
- How would you build a performance regression detection system that scales across multiple services?Senior
Standardise the test-and-report pipeline across services: a common schema for result storage, per-service baselines versioned with code,…
- How do you define and maintain performance SLOs across multiple services in a growing organisation?Lead
Define SLOs at the service level, derived from user-facing business requirements rather than infrastructure capabilities. Review and upda…